Most cap-sensitive, cast, solid explosive compositions usable as primers are made from molecular explosives such as PETN, TNT, RDX or combinations thereof such as pentolite and composition B. These molecular explosives products have relatively high densities (1.60 g/cc or greater) and are formed from liquid melts at high temperatures. The high temperature liquid melts are poured into containers and allowed to cast upon cooling to the desired solid form. The melting, pouring and casting steps involve inherent hazards due to the high temperatures involved and the presence of molecular explosives. Recently, a novel cast, solid explosive composition was invented that allows mixing, pouring and casting of non-explosive ingredients to occur at ambient temperatures. The ingredients simply are admixed at ambient temperature to form a slurry that can be poured into containers and allowed to cure with time into a cap-sensitive, cast, solid form. (See co-pending U.S. Ser. No. 08/201,341.) In fact, when the non-explosive ingredients first are mixed together at ambient temperature, the mixture typically is not cap-sensitive, but upon curing, also at ambient temperature (except for the temperature rise due to heat of hydration and solvation as described below), the mixture casts and increases in sensitivity to become cap-sensitive. The inherent safety advantages of these compositions are obvious. Not only are non-explosive ingredients admixed at ambient rather than elevated temperatures, but also the composition increases in sensitivity only after the mixing step and simply upon being allowed to cure. These recent compositions comprise sodium perchlorate oxidizer salt, a polyhydric alcohol of low volatility such as diethylene glycol, and a small amount of water. The present invention is an improvement to these novel compositions, which hereafter will be referred to as "cast compositions."
Even though the cast compositions remain cap-sensitive and detonable at high densities (1.78 g/cc or higher), as do molecular explosives, the cast compositions tend to require greater run-up distances to reach terminal detonation velocity than molecular explosive-based compositions, which have short run-up distances. (Run-up distance is defined as the distance along the length of a cylindrical explosive charge that is required for the charge to reach its steady state or terminal detonation velocity, as measured from the point of initiation.) Also, these cast compositions have comparably higher critical diameters (unconfined) than do molecular explosives. (Critical diameter is defined as the minimum diameter at which a detonation wave is sustained in an explosive.) Further, as the diameter of the charge decreases, the detonation velocity of the cast compositions may decrease to a level (below about 5,000 m/sec) that is unacceptable. A shorter run-up distance, a smaller critical diameter and a higher terminal detonation velocity are preferred for booster and seismic charges. These characteristics are particularly important for small size (less than one pound) small diameter boosters or primers or minihole seismic explosives.
Another problem with the cast compositions as compared to molecular explosives involves impact sensitivity. The cast compositions can be more sensitive to impact initiation, depending on the impact stimulus, than molecular explosive products, and this difference in impact sensitivity can be a safety concern.
In summary, a need exists for the cast compositions to have shorter run-up distances, smaller critical diameters, higher terminal velocities in smaller diameters, and reduced impact sensitivity. The present invention satisfies these needs.
It has been found in the present invention that by adding a relatively small amount of microballoons and dispersing them throughout the cast composition, not only is the run-up distance decreased to a relatively very short distance (.ltoreq.50 mm), but also the critical diameter is decreased to .ltoreq.0.5 inches. In addition, the impact sensitivity (to rifle bullet and air cannon initiation) is significantly reduced when a small amount of microballoons is added. This effect is surprising since normally the addition of microballoons or air voids to an explosive, even a molecular explosive, increases the detonation (and impact) sensitivity of the charge, particularly in charges having small critical diameters.
A possible explanation of this phenomenon in the present invention is that the microballoons act as "energy absorbers" in localized, decoupled regions within the explosive matrix, where the energy created by an impact is dissipated or interrupted before significant reaction of the ingredients takes place. The fact that the detonation run-up distance also is decreased seems to indicate that initiation sensitivity and impact sensitivity of these cast compositions occur by different mechanisms.
With respect to initiation sensitivity, once the detonation process has been initiated by a brisant, localized shock energy source (blasting cap), the microballoons facilitate propagation of the detonation wave such that it reaches its terminal velocity more quickly (shorter distance). The microballoons perform this function by serving as hot spots (adiabatically compressible gas pockets). For impact sensitivity, however, the microballoons prevent transition to detonation in the product by dissipating or interrupting the relatively low energy imparted by the impact source. In contrast, molecular explosives-based products tend to have excellent detonation properties (such as minimal run-up distance, small critical diameters and high velocities even in small charge diameters) at higher densities and do not need the presence of hot spots to help propagate the detonation wave.
Another property of the present cast composition is that the curing or casting time generally is reduced when plastic or glass microballoons are employed. This is advantageous since the overall manufacturing time can be reduced.
All of these described benefits combine to make the cast compositions useful for small booster (less than one pound) applications or minihole seismic explosives (one-third pound) applications, in which the products have short charge lengths and small diameters.